Photos of Mt. St. Helens on the 15th Anniversary of
the Eruption (18 May 1995).

A Geological Guidebook to Dante's Peak

For the General public

Our Commentary:

First - go see the movie (we thought
it was OK). Dante's Peak is an
all-too-rare opportunity for you to enjoy a good adventure/"nautural
catastrophe" movie, and at the same time, get a feel for how geologists
(and other scientists) work. You can also learn some science along the
way. Of course, the writers, producers and directors did exercise
quite a bit of artistic license in the movie, as they
should; otherwise, our heros and heroines would have been killed off as
soon as the action started! So, you will find some "scientifically
imprecise" embellishments in the movie, and we find this quite
understandable.
We can think of no
movie/television series that portrays scientific concepts and scientific
practice as well as this flick does. As we said, our aim here is to
point out the scientifically
related plot elements that we think are accurate and realistic, and those
wherein the writers stretched things a bit to make the movie exciting.

OK, let's get started:

The Movie is set in the
Cascade Range of the Pacific Northwest, U.S.A. Dante's Peak is, of
course, not a real Cascade volcano, and although Idaho (where some
scenes are shot) is adjacent to states
with Cascade Volcanoes (Oregon and Washington), and another state with an
active magma chamber (Wyoming-magma exists under Yellowstone National Park),
there are no Cascade (or any other active or dormant volcanoes) in Idaho
(there is, however, plenty of evidence of past volcanic activity, as
there is
in most parts of the world including many of the 50 United States).
Anyway, as a keen observer has pointed out to us, the Mayor's LandCruiser
has a Washington State licence plate, and Washington does have
active volcanoes.

Wait! We need to start at the beginning (well, not exactly the beginning...):

When
Volcanoes in the Cascade Range and Alaska erupt, they frequently do so
explosively and produce
pyroclastic flows, ash
falls and "mud" or debris flows (lahars).
According to the USGS, "Lahars destroyed houses, bridges, and logging trucks
during the May 1980 eruption of Mount St. Helens, and have inundated other
valleys around Cascade volcanoes during prehistoric eruptions. Lahars at Nevado
del Ruiz volcano, Colombia, in 1985, killed more than 23,000 people." Near a
volcano, the falling volcanic ash is quite heavy (high density), and the
newspaper used as a proxy for volcanic ash in the movie looked more like snow
(low density) as it fell. For geologists who have seen the movie, the
hot, runny lava, seen issuing from the volcano, is the most bothersome
issue. Generally, runny, fast-flowing lava (basalt) erupts from Hawaiian or
"shield"
volcanoes; but we understand that this makes for a more exciting movie. Lava
flows at Cascade volcanoes are usually thick, and rarely move far from the vent
(for example Mount St. Helens Dome: see the center photo at the top of this
page: the dome
is a mound of rather thick lava that has partially or wholly solidified) unlike
the
Hawaiian-type flows and lava fountains shown in Dante's Peak. Stratovolcanoes
like the Cascades do not usually produce pyroclastics and lava in the same
eruption. Also, lava is VERY hot (over 1500 F), and most flammable material
(rubber, wood, people) brought near the lava would burst into flame. The
radiative heat alone is sufficient; flammable materials do not even have to
touch the lava. We think a car would last only seconds on a lava flow before it
would burst into flames, consuming the occupants. It is worth pointing out here
that
lava itself does not burn (most of the constituents of lava are already in their
"most oxidized" state), and what we see burning around lava flows consists
mostly
of grass, houses. trees, shrubs, animals, etc.

We noticed that the lake near Grandmother's house becomes acidic quite fast
in the movie, but hey, you need to pack a lot into a movie these days to draw a
crowd. There are very acid
lakes around volcanoes and yes, you would not want to swim in them...
and
yes metal parts can corrode in acid lakes; a thin metal cable might
dissolve on a movie timescale. The acids may be sulfuric and hydrochloric acid,
plus others, and the acidity (roughly measured by how low the pH
is) can be quite high, hundreds to thousands of times as acidic as lemon juice
or vinegar, or EVEN Coca Cola! A pH on the order of 0 to 2 would not be
unexpected.

The hot springs in any geothermal area (e.g. Yellowstone; Long Valley, CA) can
turn on or off, or change temperature abruptly, but these changes are usually
related to rapid changes in the "underground plumbing" (the system of cracks and
fractures present in underground rock that allows hot or cold underground water
to flow from one spot to another brought about by earthquakes that are
usually not directly related to the magmatic activity in the area). Since most
undergound hot water flows through fractures that may be active as faults,
changes in the fracture/fault system during earthquakes can redirect flow, or
increase or decrease it.
However, hot springs (like the one that parboils a "couple in love" in the
movie), do not turn "red-hot" when they heat up(!) as shown in Dante's Peak.
Increased steam discharge and bubbles would be likely if the temperaure
increased. So, changes in hot spring activity suggest recent strong earthquakes,
with or without an impending eruption.

Trees and animals killed by magmatic carbon dioxide? Strange as it seems, yes,
carbon dioxide gas released from undergound magma chambers can accumulate in
soil and kill plants and trees or suffocate animals. Trees have died by having
their root environments flooded with carbon dioxide. Note that sugars, formed
by
photosynthesis during the day, are USED by the plants as food at night (plant
respiration), and plants need oxygen just like we do at those times! Ground
dwelling animals (squirrels, ants, etc.) can also be killed as carbon dioxide
displaces oxygen gas-bearing air in soil pore spaces and subterranean critter
dwellings. About 100 trees have
been killed around Mammoth Mountain, CA since 1989. USGS scientists have
suggested that the gas is bubbling out of magma that lies a
few miles under Mammoth Mountain. Note, however, that the magma is not currently
moving toward the surface; so you decide if Pierce Brosnan is correct in
predicting an eruption at any moment!

Volcanic tremors are different from normal earthquakes, but you
probably can't feel the difference - Geologists can see the difference on a
tracing (a
seismogram) made by an instrument called a seimograph, that detects
the amount of ground shaking produced by an earthquake. Volcanic
quakes usually register a 4-5 or less on the Richter Scale; therefore, they
usually do not produce the type of destruction seen in the town of Dante's Peak
upon the start of the eruption. The destruction in the movie suggests a 6-7
magnitude temblor, although the amount of destruction is a complex function of
distance to the quake, duration of the earthquake, the type of rupture, the
nature of the soil/rocks in the area, and the type of building constrcution.

Regarding the suddeness of the activity of Dante's Peak, volcanoes can become
restless and erupt with only days to weeks warning. According to a USGS
website,
"The first steam eruption at
Mount St. Helens on March 27, 1980, was preceded by
only 7 days of intense earthquake activity. The climactic eruption, on May 18,
followed seven weeks later. An eruption of Redoubt Volcano
in Alaska on December
13, 1989, was preceded by only 24 hours of intense earthquake activity. But
other volcanoes have been restless for months or years before an eruption
occurred, and sometimes a period of unrest doesn't produce an eruption at all."
So, the suddenness of the eruption of Dante's Peak was not was not a dramatic
effect.